Electroencephalogram (EEG)

What Affects the Test

Reasons you may not be able to
have the test or why the results may not be helpful include:

Moving too much.

Taking some medicines, such as those used to treat seizures
(antiepileptic medicines) or sedatives, tranquilizers, and
barbiturates.

Not eating before the test.

Drinking coffee, soda, tea, or other foods that contain caffeine before the test.

Being unconscious from severe drug poisoning or a very low body
temperature (hypothermia).

Having hair that is dirty, oily, or covered with hair spray or
other hair preparations. This can cause a problem with the placement of the
electrodes.

What To Think About

If the doctor thinks that a person has epilepsy but the EEG is
normal, the technologist running the EEG test may have the person look at a
flashing light (photic stimulation), breathe fast and deeply
(hyperventilation), or sleep during the test. These techniques sometimes show
epileptic EEG patterns that did not show up at first. If epilepsy is suspected
after an initial EEG, the doctor may repeat the EEG more than once.

An EEG done during a seizure will almost always show abnormal
electrical patterns. This makes an EEG useful when a doctor thinks that a
person is having psychogenic nonepileptic seizures (also called pseudoseizures), which have no physical
cause but can be caused by stress, emotional trauma, or mental illness.
Psychogenic seizures do not cause abnormal electrical activity in the brain and
will not show abnormal EEG results.

Other tests that may also be done include:

Video EEG. Video EEG records seizures on videotape and on computer so
that the doctor can see what happens just before, during, and right after a
seizure. This test can be very helpful in finding the specific area of the
brain that the seizures may be coming from. It is also helpful in diagnosing
psychogenic seizures, which may look like real seizures but do not affect the
electrical activity in the brain. Video EEG may be used short-term or
long-term:

Short-term monitoring is done on an outpatient basis and
may last up to 6 hours.

Long-term monitoring is done in the hospital and may last
3 to 7 days.

Brain mapping. Brain mapping is a fairly new method that is very similar to
EEG. With electrodes placed on the person's scalp to transmit the brain's
electrical activity, a computer makes a color-coded map of signals from the
brain. It is sometimes done to find a specific problem area in the brain that
has already shown up on a regular EEG. Doctors are still not certain how brain
mapping could be best used.

Ambulatory EEG monitoring. In ambulatory EEG monitoring, the person is able to move
around, and the test allows for long periods of time in recording of electrical
activity in the brain. Fewer electrodes are attached to the person, and the
person carries a small, portable recording unit. The recording may last for a
full day or more, and the person is allowed to leave the hospital. Ambulatory
EEG monitoring is not as accurate as a regular EEG.